Hitherto, as a vibration damping rubber, a rubber obtained by blending carbon black as a reinforcing material in natural rubber has been used. In recent years, in the market, vibration damping rubbers, particularly, vibration damping rubbers for automobiles have been required to be decreased in dynamic magnification (“dynamic spring constant”/“static spring constant” ratio).
In order to decrease a vibration damping rubber in dynamic magnification, it is important to heighten the dispersibility of carbon black in natural rubber. Hitherto, a method has been adopted in which the dispersibility of carbon black in natural rubber is heightened by using a kind of carbon black having a large particle diameter as carbon black. However, this method tends to damage the vibration damping rubber in endurance.
In general, together with a vulcanizing agent containing sulfur, a vulcanization accelerator is blended in a rubber composition in order to shorten the vulcanizing period, lower the vulcanizing temperature and decrease the amount of the vulcanizing agent. A compound for activating this vulcanization accelerator and making the accelerating effect thereof higher is a metal oxide, a typical example thereof being zinc oxide (zinc flower). Patent Document 1 listed below states that a vibration damping rubber decreased in dynamic magnification can be obtained by vulcanizing a rubber composition obtained by blending a specified amount of zinc flower made into fine particles in 100 parts by weight of natural rubber. However, the inventors have made eager researches to find out that from the viewpoint of a decrease in the dynamic magnification of vibration damping rubbers, there remains room for further improvement in the technique described in this patent document.
Patent Document 2 listed below describes, as a raw material of a pneumatic tire, a rubber composition containing SBR and a complex zinc flower in order to improve various physical properties when the composition is made into the pneumatic tire (for example, the operability and ride quality of an automobile, and the abrasion resistance of the composition) while the thermal aging of the composition is restrained. However, the rubber compositions described in these documents are not intended to be used for vibration damping rubbers. The documents much less state or suggest that the use of the rubber compositions is effective for decreasing the dynamic magnification.
Patent Documents 3 and 4 describe methods for producing complex zinc flower particles, and describe techniques of blending the resultant complex zinc flower in a rubber composition.